Suppose you built the Tower of Babel--successfully. Suppose you climbed it all the way up: past the moon, past the sun, past the stars, to the roof of the world. Suppose you found a ceiling at the top of the sky, the floor of the Vault of Heaven. And suppose you managed to tunnel through that ceiling/floor. What would you find inside the Vault of Heaven?

The correct answer--there is only one, I think--won Ted Chiang a Nebula for "Tower of Babylon" in 1990. Can you guess it?